1. Field of the Invention
The present invention generally relates to manifolds for conveying fluids, and more particularly, pertains to heated distribution manifolds for conveying molten plastic.
2. Description of the Related Art
Modern plastic injection molding techniques involve forcing molten plastic at high pressure into a mold. Modern plastic materials are temperature sensitive, and typically have narrow thermal processing ranges. If the material being injection molded is too cool, it may not be molded properly. If it is too hot, it may decompose. Moreover, many thermoplastic materials have a limited lifespan when kept at their optimal molding temperatures.
In injection molding, the usual source of molten plastic is a screw/ram-type injector. The molten plastic is guided from the source to the mold through a suitable manifold and drop structure. Economies of scale may favor the use of more than one mold, and in such cases the manifold structure may contain a number of passageways leading through drops to the individual mold cavities.
The plastic leaving the screw extruder or other plastic source is molten, and flows under pressure. It is important to regulate the temperature of the plastic melt carefully, since the plastic temperature directly and markedly effects the molding process. If the molten plastic is too cool, it may not properly fill the molds, or the plastic may not take on its desired shape in the mold. If the plastic cools even further, it will solidify, and plastic flow through the system will stop. Now, manufacturing time will be lost while the system is cleaned.
Accordingly, it is known to heat the manifold structure which carries the molten plastic from the ram/screw extruder to the molds. One way of heating the manifolds is to embed electrical resistance heaters in the manifold structure, as described, for example, in U.S. Pat. Nos. 4,219,323, 4,256,140, 4,609,341, 4,688,622, and 5,032,078. Alternatively, U.S. Pat. No. 3,704,723 suggests providing cylindrical non-directional radiant energy sources in bores running through the manifold.
Manifolds which are heated by resistance heaters embedded in the manifold are prone to "hot spots". These "hot spots" are caused by the way in which the resistance heaters are constructed. Resistance heaters typically consist of a piece of conductive wire wrapped around an insulating core. That assembly is covered by a suitable material such as ceramic, which is encased in a metallic jacket. Since the conductive wire becomes extremely hot when current passes therethrough, and to avoid short circuits, the core and cover are usually made of ceramic material.
Since the metallic jackets are difficult to machine accurately, available heaters do not have constant diameter outer surfaces. Variations of 0.020"/foot are typical. It is these variations which cause the "hot spots".
Because some parts of the heater jacket are narrower than the diameter of the bore in which the heater is placed, not all of the surface of the heater contacts the manifold block. As a result, heat generated by the resistance heater only passes into the manifold block at those points where the outer surface of the resistance heater jacket contacts the inside of the surrounding bore in the manifold. However, since the entire heater is generating thermal energy, the amount of energy which is being transferred where the heater touches the block may exceed the thermal conduction ability of the block. Thus, these regions will heat up dramatically, forming "hot spots" within the manifold.
Such "hot spots" are undesirable because they can overheat the molten plastic flowing through the manifold. This is a serious problem, since many plastics used in injection molding have limited thermal stability, and they will decompose if heated excessively. Depending on how a plastic decomposes, it may form carbon or other material which will contaminate the product and possible plug the manifold. A far more serious problem arises if the molten plastic decomposes into substances which react with the remaining plastic or the manifold itself. Violent explosions may occur, possibly destroying the injection molding system. For example, if some polyvinyl chlorides (PVC) reach too high a temperature, or are held too long at an elevated temperature, they can degrade and form, among other things, hydrochloric acid (HCl). This hot CL can react with the manifold and mold bodies, and cause a catastrophic explosion.
The desirability of avoiding "hot spots" is therefore well-known. The prior art suggests various ways to avoid forming such "hot spots". One approach is to regulate more accurately the temperature of the resistance heaters. For example, in order to more precisely control the temperature of the manifold, U.S. Pat. No. 4,292,018 suggests using several separately-controlled resistance heaters. This structure is said to allow the molten plastic contents of the manifold to be maintained at the optimum temperature, without burning.
Another approach is to improve the fit between the heater and manifold. U.S. Pat. No. 5,032,078 states that the heaters used to warm the manifold have cylindrical outer surfaces that provide a good fit in the matching grooves in the manifold, so that heat will be efficiently and uniformly transmitted to the manifold body.
In an effort to improve contact between the heater and the manifold, U.S. Pat. No. 4,761,343 casts conventional cartridge heaters in copper, the copper in turn being contained within a machined tool steel outer body. However, in practice, thermal cycling of the heaters leads to the formation and enlargement of voids in the copper. Ultimately, when these voids become large, heat transfer is impeded and again, hot spots will form.
However, none of the foregoing approaches toward heating a plastic injection manifold are fully-satisfactory. For example, when electrical cartridge resistance heaters are used, they produce hot spots which can cause plastic degradation, and possible manifold explosions. In addition, it is quite difficult to remove these cartridge heaters when they fail, because with time the hot cartridge body diffuses into and so bonds to the manifold body. Attempts to avoid hot spots through improving heater-manifold contact by pouring molten metal around the heaters are also unsatisfactory, since such heaters are exceedingly difficult to service. Moreover, such manifolds eventually develop hot spots, as over time voids form in the metal surrounding the heaters. Other approaches involving circulating hot liquid through the manifold are also unsatisfactory; the manifold structure is quite complicated, the hot liquid is dangerous, and such systems are subject to failure, for example, where the hot liquid flows into and out of the manifolds.
Since none of the foregoing techniques and devices do a fully-satisfactory job of preventing the formation of "hot spots" in a manifold, there exists a genuine need for a way to reliably and uniformly heat a plastic distribution manifold.